Update git eclass from x11 overlay.
FEATURES
* git submodule support
* LIVE_FAIL_FETCH_IF_REPO_NOT_UPDATED usage
- this makes build die when git repo is up-to-date and nothing new
has been done in it. Quite lovely with --keep-going.
* specific per overrides of TREE/BRANCH/REPO
- just define ${PN//-/_}_LIVE_TREE/BRANCH/REPO in enviroment and you can
override in ebuild values.

EAPI2fy git eclass. Make it eclassdoc compatible (at least i hope :])
There are new elog informations which are requested by upstream often.
Note that this elog can be supresed by setting EGIT_QUIET="ON" in make.conf or somewhere else in the env. :]

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